Aluminum (or aluminium to our UK friends) is one of the most useful metals that are commonly available. Unlike other metals such as iron, copper, and the like, aluminum has been used in large quantities only fairly recently. Actually, alumimium is the better name, because it is in keeping with the naming of most metallic elements with the “ium” ending. However, we shall use the US term. Interestingly, the brilliant British chemist Sir Humphrey Davy called it aluminum, but he never produced the actual metal. In addition, his first name for it was alumium, and folks from My Little Town who were older used that name! That really should be the systematic name for it.
Aluminum compounds have been known for centuries, but the free metal only since around 1825 and even then in an impure form. It was not until very late in the 19th century that aluminum was produced on a large scale, using a process that is essentially identical with the process being used even now.
Before the metal itself was first isolated, aluminum compounds were important items of commerce with many uses. Aluminum oxide in the form of corundum has been used as an abrasive for centuries, whilst two other forms of this same oxide are ruby and sapphire. Alum, a complex potassium and aluminum sulfate, has been used since antiquity for several different applications, and today we see it in everyday life as those styptic pencils for razor nicks and as an ingredient to make pickles crisp.
The simple aluminum sulfate is used in huge quantities for water treatment, where it causes suspended solids to flocculate, or to make the very tiny solid particles group together so that they settle out of the water.
One nice thing about aluminum compounds is that they are very common and relatively cheap. Another advantage is that aluminum is not very toxic (although this is somewhat controversial, and we shall get into that in a bit), so can be used without concern about toxic effects on the environment or on people. The reason that aluminum compounds are cheap is that aluminum is the most common metal in the crust of the earth, and the third most common element in the crust, after oxygen and silicon. Note that these figures are on a mass basis, not a molar basis.
Before the modern process of producing aluminum came into existence, it was prepared by reacting either sodium or potassium metal with aluminum chloride. Both the alkali metals are expensive and very reactive, and aluminum chloride reacts violently with water. The end products are aluminum metal and either sodium or potassium oxide. The aluminum is in the form of a powder, so it has to be melted and cast into ingots for further use. At the time that the Washington Monument was finished, aluminum was chosen as the top cap for it, and a about a six pound casting was prepared. That was the largest solid piece of aluminum on the planet at the time, and was worth about the same as a similar piece of silver. At today’s silver prices, it would be worth around $2500!
Around 1886 the same process was developed in the the US by a very young man named Charles Martin Hall, and in France by and equally young Paul Héroult. Both were only 23 years old when they came up with the same basic process. It is very coincidental that both were born in 1863, discovered the Hall/-Héroult process in 1886, and both died in 1914. In this process, highly purified aluminum oxide (alumina) is dissolved in molten cryolite (a sodium aluminum fluoride) at around 1000 degrees C. This is done by passing an extremely heavy current through the cell containing the cryolite and alumina, and this current also causes the metallic aluminum to be produced.
The cell for making aluminum is called in the trade a “pot”, and it is essentially an insulated box of steel lined with ceramic both as a thermal and electrical insulator. A large carbon electrode forms the floor of the box, and it is at a negative potential of around five volts with respect to the huge carbon anodes that are held by a crane at the top. As the current passes, the aluminum in the alumina is reduced from the 3+ oxidation state to the 0 oxidation state, and the oxygen in it is oxidized from the 2- state to 0. This gaseous oxygen burns the carbon anodes as it is formed, so they have to be lowered constantly to maintain contact. This also produces a lot of carbon dioxide, and aluminum refining is a major contributor of carbon dioxide to the atmosphere.
The anodes eventually have to be replaced as they burn, either by lifting the spent ones out and dropping in new ones, or by adding heavy pitch to a container at the top of the anode that, due to the heat produced, chars to compact carbon. I said that the currents used were extremely heavy, so how does around 350,000 amperes sound? That is an average figure, and some cells operate at higher currents than that. Think about 350,000 amperes for a minute. The main breaker for the electrical service in my house is rated at 150 amperes, and I doubt that even with EVERYTHING running I could pull that much current. A single aluminum pot thus draws something like 2300 of houses like mine running full tilt! Most facilities run numerous cells simultaneously, so the current draw is almost unimaginable.
It is for this reason that aluminum refining is usually done near sources of vast amounts of electricity, often hydroelectric. Plentiful and cheap power is essential, and unless hydroelectric power is used, as in the UAE where abundant natural gas provides the electricity, this forms another source of carbon dioxide release in connexion with aluminum production. But carbon dioxide is only the start. It turns out that aluminum production is quite the environmental nightmare.
First, enormous amounts of bauxite, the ore for aluminum, have to be mined. For example, Australia alone produced 62 million metric tons of bauxite in 2005. For 2008, world production of bauxite amounted to 205,000 million metric tons! That is a BIG hold in the ground! Arkansas used to produce quite a little bauxite, and I well remember the numerous strip pits in central Arkansas from bauxite mining.
Once the bauxite is mined, it is ground and mixed with strong sodium hydroxide solution to form sodium aluminate. Sodium aluminate is soluble in a hot solution, whilst most of the impurities in the ore are not. Once those are filtered away, the solution is cooled and the insoluble aluminum hydroxide precipitates. This is washed to remove residual sodium hydroxide, carbonate, and silicate (the carbonates and silicates from the rock) and then strongly heated to convert it to the oxide. Both the material filtered off and the spent solutions have to be disposed of properly and also present an environmental challenge. Since the best bauxites are only around 50% alumina, and many much poorer, the solid waste is a real disposal challenge. By the way, producing alumina in this was is called the Beyer Process.
Another set of environmental challenges have to do with operation of the cells. One of the byproducts of cell operation is hydrogen fluoride from the cryolite, and this has to be captured, usually by passing the exhaust through a scrubber containing sodium hydroxide where it is trapped as sodium fluoride. Another problem is disposing of the cells when the cathode at the bottom finally is worn away by the aluminum mechanically damaging it. The problem is the “pot liner”, which is extremely high in fluoride, in the high per centage levels. Only a few hazardous waste landfills are permitted to accept pot liner, and it is some bad stuff.
It takes around 15 kilowatt-hours to produce a kilogram of aluminum, and that is about my electrical use per day in winter. So all the power that I use in my house would only produce a little over two pounds of aluminum per day! Now, this aluminum is only around 99% pure, and that is not pure enough for many purposes. Another process, the Hoope Process, is used to improve the quality to 99.99%. In this process, molten Hall aluminum in constantly added to a cell containing three liquid layers: the bottom, in contact with a carbon electrical connecter consists of molten aluminum, copper, and silicon. The middle layer is composed of cryolite and barium fluoride, and the top layer is purified aluminum that is tapped from time to time. There are also carbon anodes at the top. As the impure aluminum is oxidized at the anode, it forms Al3+ ions that migrate to the cryolite layer, but the impurities stay at the bottom. Then these ions exchange for ones in the cryolite, and the cathode at the top of the cell reduces those back to metallic aluminum. Since the aluminum layer is the least dense, the pure aluminum floats. But it would not float except for the barium fluoride, which makes the cryolite layer more dense than it otherwise would be. Barium is another environmental hazard as well.
Now that we have made the aluminum, what do we do with it? Just about everything! Except for steel, we probably touch and use more aluminum than any other metal, yet few of us have ever seen pure aluminum! That is because pure aluminum does not have very good mechanical properties. It is soft, will not accept heat treatment, and is mainly used for electrical transmission since it, although not as good a conductor as copper, is lighter and less expensive making it more economical than copper. If you are unfortunate enough to have a house with aluminum wiring, as I have, then you have seen pure aluminum. Alloying the metal reduces its conductivity too much.
Aluminum is widely used in the transportation industry to reduce weight in rolling stock. It can be used in a wide variety of applications, from body panels, to wheels, to many under the hood components, and even engine blocks. It is essential for the aerospace industry where the high strength and light weight of many of its alloys allow for lighter aircraft than other materials.
Aluminum is widely used in construction fir things like windows and other extruded pieces because it is easy to work and can be treated to take a good finish. Vinyl has replaced it in large part due to the tendency for aluminum to dent when struck, but there is still a lot of aluminum siding around.
Countless consumer products are made of aluminum, many of them having to do with the kitchen. Everything from pots and pans, skillets, drinking glasses (especially for poolside use), and the universally used aluminum foil and aluminum cans to potato chip bags are common. The chip bags are interesting because they are mostly plastic, but coated with aluminum by vacuum deposition so that an oxygen barrier is put in place. Otherwise, the chips would go stale in the bag rapidly because of their high fat content.
I guess that I should tell you why aluminum foil has a shiny side and a flat side, except for the really thick kind. Because foil is so very thin, it is difficult to calibrate the rollers that do the final squeeze to those dimensions, because we are talking about a thickness of only around 22 micrometers thick. It is almost impossible to calibrate rollers to make such a thin piece. Thus, on the last rolling, two pieces of aluminum are fed through the rollers after a release compound is added to keep them from cold welding together. It is much easier to make the rollers around 45 micrometers apart, and the result is that the flat side results from the contact of the two aluminum pieces and the shiny side is the result of the highly polished roller being in contact with the “outside” of the metal.
Aluminum foil is often referred to as “tin foil” because tin used to be used for the same purposes. There is no tin, except as an accidental contaminant, in modern aluminum foil at all. The only memory that I have of actual tin foil was the old tubes of things like toothpaste, and those are almost either plastic or aluminum now. Tin is way too expensive to be used as foil any more.
I used to use aluminum when I ran the pyrotechnics development facility for the Army. It is an excellent fuel for pyrotechnics in the form of a fine powder, and powdered aluminum is also used in metallic paints. You have to be careful handling it, because it is treacherous in the form of uncoated powder. Usually aluminum powder is shipped coated with mineral oil that has to be removed with a solvent before pyrotechnic use, but can be left on for paint use.
Why is this so? Aluminum is actually EXTREMELY reactive chemically. You would not think so, but it is. The reason that it seems to be so nonreactive is that aluminum instantly, when exposed to air, forms an extremely thin film of aluminum oxide that, unlike iron oxide, has a larger volume than the underlying metal AND is tough. This oxide coating protects the highly reactive metal from further oxidation. In the case of anodized aluminum, a special treatment produces a much thicker oxide coating than natural ones, and these coating can also be made porous enough that they will accept solvent soluble dyes, thus coloring the items without paint. Since these oxide coatings are so hard (remember, aluminum oxide is used as an abrasive) they wear much better than paint.
This oxide coating is a real problem with household wiring, however. In addition to being hard, aluminum oxide is good electrical insulator. When a wire is stripped, it instantly gets this coating. When screwed down into a switch, for example, this partially insulated connexion has a lot more resistance than it should, causing it to heat when a current is passed. When the switch is turned off, the connexion cools. The repeated expansion and contraction of the wire caused by thermal cycling, along with the softness of pure aluminum compared to copper, loosens the connexion after time passes, making it so poor that fire is likely. I helped my neighbor replace a nonfunctioning baseboard heater in house a couple of weeks ago (like mine, his has aluminum wiring) and immediately saw the source of his problem: the connexion failed because of aluminum corrosion and creep. He was lucky that the connexion failed “open” and did not ignite. When we replaced the heater, I came back and got a special anticorrsion paste for just this purpose. When we reconnected it, I took very fine sandpaper and coated it with the paste, then carefully removed the oxide coating by abrasion. The paste is an oxygen barrier, so it reduces the amount of oxide coating formed. I also filled the wire nut with the paste before I joined the house wiring to the new heater. By the way, I almost backed out of buying this house when I saw the aluminum wiring, but the seller at my request replaced every outlet and switch in the place with ones rated for aluminum wiring. Even then, I used the anticorrosion paste and sandpaper on every connexion in the place. By the way, outlets and switches rated for aluminum are stamped “CO/ALR”. Just be sure to use the paste and sandpaper. I now have to check all of my circuit breakers to make sure that they are stamped “CU/AL”, the standard for them. I shall replace them one by one if they are not, and be sure to use the sandpaper and paste.
Because of the technology to make connexions for high voltage power transmission, aluminum creep is not a problem, and in many cases heavy, stranded aluminum lead in cables are still within code for delivering electricity to the electric meter. From that point on, current code requires copper or a new aluminum alloy that does not suffer from the disadvantages of pure aluminum. Since the conductivity is lower, the wire gauge has to be larger but extensive testing has shown when properly installed is safe. The reason that aluminum was used in the first place is that copper got high in cost in the mid 1960s and since aluminum was used for high voltage transmission, the people who write the code accepted it without enough experimentation. I use the term “pure” aluminum, when in fact the alloy used for this wiring was alloy 1350, which specifies at least a 99.5% aluminum content with only maximum amounts of contaminants. In any event, people with aluminum wiring are living in a house at their own risk unless they take the proper precautions. I spent over 40 hours of research before I satisfied myself that my wiring is now safe, and just tonight found the problem with the circuit breakers.
Finally, I promised to discuss some of the health effects of aluminum. I will go out on a limb here and state that for the vast majority of people under ordinary conditions aluminum has no adverse health effects. There may be some very rare individuals who have an allergy to aluminum, but they must be very rare indeed because you can not get away from aluminum! Remember, it is the third most common element in the Earth’s crust, so you are exposed to natural sources of it each and every day just from dirt. Aluminum hydroxide used to be a very common ingredient in OTC antacids, but most have reformulated and consist of calcium carbonate and perhaps magnesium hydroxide. Those were probably the most common sources of high aluminum consumption. Next are the antiperspirants that contain aluminum salts, but those are not very well absorbed.
It may be that because aluminum is so ubiquitous that biology evolved to ignore it. There is no living thing known that requires aluminum, and unless present in extremely high levels, there is no living thing that is harmed by it. By high levels I mean levels so high that there is so much Al3+ ion that it interferes with the normal uptake of other, required ions, such as potassium. Some plants get stunted in acidic soils that are high in aluminum, but that seems to be due to its interference with uptake and metabolism of other required ions.
You are not going to get high doses of aluminum by drinking from aluminum cans. In addition to the hard and impervious aluminum oxide coating, all drink cans are lined with a synthetic material that prevents contact betwixt the aluminum and the contents of the can. There is probably more hazard from the Bisphenol A in the liner material than from the aluminum. Likewise, cooking in unlined aluminum vessels is not that much of a problem because of the oxide coating which is pretty robust unless very highly acidic or basic foods are cooked in them. I do not cook down tomatoes in unlined aluminum, but that is mostly because I do not have any unlined cooking equipment. I still use foil to wrap vegetables, and do not think that it has caused me any harm.
There has been the hypothesis that aluminum intake and Alzheimer’s disease have some correlation. There are not really any data to support that, and I have a couple of personal anecdotes to support the noninvolvement of aluminum. My grandmum, who lived to be 100 and a half years old, was cogent and communicative until the last six months of her life. Except for her cast iron skillets, she cooked almost everything that she ate in uncoated aluminum vessels. That includes everything boiled, everything braised and roasted (she had a big aluminum roasting pan with a lid), and she used foil on EVERYTHING! The other example is my grandfather, who did pretty much the same and was mentally quite alert until the stroke that took him when he was 91 years old (in 1968, that same stroke today probably could have been treated and he would have lived longer). I know that these are only two personal experiences, but to me they are pretty significant.
My advice is simply not to worry about aluminum. Just from dust and dirt, you take in a LOT of it, and if it were that bad for you we would be keeling over left and right. Now, I know that some of you will point out the fact that some Alzheimer’s patients have elevated aluminum levels in their brain plaques, but that is not necessarily a causal relationship. With the data available, it is just as likely that increased aluminum content is due to the already abnormal metabolism rather than increased aluminum levels causing the abnormal metabolism.
Well, you have done it again! You have wasted many einsteins of perfectly good photons reading this tinny piece. And even though Rick Santorum realizes that he does not have a chance at the nomination when he reads me say it, I always learn much more than I could possibly hope to teach by writing this series, so keep those questions, comments, corrections, and other feedback coming! Tips and recs are also always welcome. I shall stay here tonight as long as comments warrant, and shall return tomorrow around 9:00 PM for Review Time.
I wish each and every one of you a very Happy New Year, and a joyful and prosperous 2012. I intend to pursue both joy and prosperity this year.
Finally, I have a request to you tech folks out there. I want to cancel my land telephone line and get a wireless instead, and keep my old number if possible. I also want to keep my high speed internet access. Any ideas on how to do that seamlessly and not loose service, and save some money?
Doc, aka Dr. David W. Smith